Conventionally, expandable polystyrene resin particles have been widely used as expandable resin particles. Due to preferable workability and heat-insulating property, the expandable polystyrene resin particles have been used for a heat reserving material for plumbing, a heat-insulating material for roofs, a member for automobiles, a heat reserving material for solar system and the like. In particular, a polystyrene molded foam used for a heat reserving material for a hot-water storage tank of a water heater is used as the expandable polystyrene resin particles which contain a flame retardant in order to promote high efficiency of equipments by reducing a heat radiation loss from the hot-water storage tank and achieve a certain criterion of heat-insulating property and further flame retardant property.
As a method for imparting flame retardancy to the above expandable polystyrene resin particles, there have been adopted a method for adding a flame retardant together with styrene-based monomer during polymerization, a method for adding a flame retardant in impregnating polystyrene resin particles with a blowing agent and the like. As the former method, there are methods as described in JP 2003-335891 A (Patent Literature 1) and JP 2002-194130 A (Patent Literature 2) and as the latter method, a method as described in JP 2007-246606 A (Patent Literature 3).